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A common question- “Why is it only recently roof cleaning has become popular?”


During the last 50 years, the UK and indeed most of Europe, has transitioned from an industrial economy to a services and commerce based society.

With fewer factories and power stations pumping out harmful and toxic chemicals into the environment, our rainfall is now much less acidic than in the past.

What some of you may not be aware of is how sensitive moss, algae and lichen are to acidic rain.

Even the slightest increase in the acidity level of rainwater is enough to kill off and prevent mosses and algae from forming.


There are many different materials that are used in the construction of a roof and some will be affected by moss growth while others won’t.

Here is a checklist detailing what is and isn’t damaged by moss growth:

Concrete tiles – No, except for a few corner cases, which are often due to the poor manufacturing techniques of older tiles. Air bubbles, small stones, sand bubbles and other objects in the cement mixture can create small holes that expand over time. Moss may grow in these holes, but it isn’t the cause of the hole. Tile imperfections are less common with newer tiles made to western standards.

Clay tiles – Yes, especially on shallow roofs. These tiles can delaminate if they get wet and then freeze, hence why clay tiles are best laid to steep roofs and kept clear of moss – so they can fully dry out.

Slates – No, not affected.

Cement – Yes, dusty, flaky and “weak” cement can be ruined by excessive moss growth.

Gutters and rainwater pipes – Yes, they can become blocked. Also, thin plastic guttering may bend, warp or snap at the brackets due to the extra weight of excessive moss, hence why gutters should be cleared frequently.

Chimney brickwork – Yes, it gets into the cement cap and between the bricks, leading to moisture ingress, when that freezes, it expands and causes cracks or damage.

Lead/Lead-work – No, this isn’t damaged by moss growth on the roof.

Is the extra weight of moss a problem on the roof? – No, this is a common claim made by some roofing and roof cleaning firms. They suggest that because wet moss is heavy, it could buckle, bow or collapse your roof. In fact, even excessive moss growth is spread so thinly over the roof that no single point or rafter ever exceeds its load capacity. Roofs are designed to hold several feet of heavy wet snow for prolonged periods of time and to hold firm against sustained gusts of wind without collapsing inwards. Thin plastic gutters may be damaged by too much moss in them though (they warp, bend or snap at the brackets).

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